This invention relates generally to a soldering apparatus and, more specifically, to a vapor phase soldering apparatus adapted for soldering printed circuit boards bearing electric chips and components temporarily mounted thereon by solder preforms.
A vapor phase soldering method is of a type in which the objects to be soldered are heated to a temperature appropriate for the soldering operation by condensing thereon hot vapors of a heat transfer liquid and is disclosed, for example, in U.S. patent specification No. 3,866,307.
Illustrated in FIG. 8 and designated generally by the reference numeral 5 is a conventional vapor phase soldering apparatus having a belt conveyor 4 for transferring through the apparatus printed circuit boards 1 on which electric chips 2 such as resistors and capacitors are temporarily mounted by means of solder preforms 3. Denoted at 6 is a vessel for containing a heat transfer liquid 7 having a boiling point higher than the melting point of the soldering preforms 3. The heat transfer liquid 7 is, for example, Florinate (SUMITOMO 3M, Inc.). Designated at 8 is a heater for heating the heat transfer liquid 7 to generate vapors 7a thereof, at 9 is an entrance opening for the printed circuit boards 1, at 10 is an entrance transfer passage, at 11 is an exit opening for the soldered printed circuit boards, at 12 is an exit transfer passage, at 13 is a cooler including cooling coils for condensing the vapors 7a, and at 14 and 15 are discharge ducts through which uncondensed vapors 7a are forcibly discharged without permitting outflows through the entrance and exit openings 9 and 11.
With the conventional soldering apparatus described above, the heat transfer liquid 7 is boiled and evaporated by the heater 8 to form saturated vapors 7a which serve to melt the solder preforms 3 on the printed circuit board 1 travelling through the vessel 6 by transferring the heat of evaporation thereto. When the printed circuit board 1 is discharged from the vessel 6 through the opening 11, the solder 3 is again solidified, securely fixing the chips 2 to the printed circuit board 1. The vapors 7a are condensed by the cooler 13 and the condensate drains back to the vessel 6.
The conventional soldering apparatus of the above-mentioned type has a problem that the transfer path is curved because the entrance and exit openings 9 and 11 are required to be provided at an upper portion of the vessel 6 in order to prevent the escape of the vapors 7a which have a higher density than air. If the openings 9 and 11 are provided at a lower positions so as to provide a straight path of travel of the printed circuit boards 1, a significant amount of the vapors 7a of the heat transfer liquid will be discharged out of the apparatus through the discharge ducts 14 and 15. Since the heat transfer liquid is very expensive, the replenishment thereof for making up for the discharged vapors incurs an uneconomically high process cost.